The exhaustion of fossil fuel reserves together with the environmental and climatic changes linked to their utilization has developed new technologies which will utilize the hydrogen as source of energy. The advantages are easily foreseeable using as energy source the sun, the renewable solar energy will be utilize to decompose the water in hydrogen and oxygen, hydrogen burns either in conventional engines or in fuel cells without pollutants emission to generate electric energy. Many technological aspects have still to be solved in order to implement this project, in particular case those referring to the transformation of solar energy into electric energy and its further use for production of hydrogen by water hydrolysis.
Presently, only 2% of the hydrogen produced comes from electrolytic processes, most of the hydrogen industrially produced comes from the hydro-reforming of fossil fuels or as industrial by-product of industrial processes such as oil refinery and PVC.
The electrolytic produced hydrogen has an high purity, but an high cog due both to the high cost of electric energy and to the low yield, i.e. low efficiency in the energy conversion from electric energy to the chemical energy.
The incentives to improve the efficiency of the electrolytic production of hydrogen are presently small: although the added value of high purity of electrolytic hydrogen would render the higher cost unimportant, such applications are rare and the use of hydrogen for the production of energy is uneconomical either for production of electrolytic hydrogen with high yields.
An improvement is expected from the continuous higher request of clean energy which foresees the use of hydrogen both for production of electric energy and for use in the automobiles industry. In the next decade the request of pure hydrogen will increase drastically, the need of more performing hydrogen production processes will be then evident, i.e. not only higher energetic yields but intrinsic safe run conditions and simple hydrogen distribution network.
In order to contribute to the development of systems which avoid the use of fossil fuels such as coal or natural gases, the choice of systems producing hydrogen from electrolysis of water is unavoidable. Environmentally friendly electric energy can only be produced using Aeolian systems, hydroelectric systems and finally using photovoltaic systems.
The energy sources of the first two systems are normally close enough to the site of further use of the electric energy whereas efficiency and quantity of electricity produced using the photovoltaic systems is higher in secluded parts of the hemisphere such as tropical and desert areas.
The photovoltaic system concentrates the solar energy and can attain up to 30% of electric conversion efficiency through the use of a dual converter, two semiconductors with different band-gaps, receiving different fraction of radiation. The produced photovoltaic electric energy can conveniently be used for the production of high purity hydrogen and oxygen by water electrolysis. The H2 stored as a metal hybrid is conveniently transported to the site of use and production of electric energy.
A major goal in electro-conversion of solar energy is the use of electricity to produce H2 and O2 of high purity using water electrolysis, transporting the produced H2 and O2 to the utilization site and recombining them in a fuel cell for the production of electric energy. Consequently in order to minimize the energy losses there is the need of developing electrolysers and fuel cells of simple geometry and high efficiency, which can be simply adapted either as electrolyser or as fuel cell.
Besides the above described system, where large size electrolysers and fuel cells are foreseen, there is a need of developing technologies suitable for use in residential power system.
Alkaline electrolyser and alkaline cell based upon the technology of the alkaline fuel cells (AFC) were the most promising. These cells have been successfully used in the Apollo project and have the highest output voltage among fuel cells; furthermore, they may be operated over wide ranges of pressure and temperature. The technology behind the electrodes has been refined in the 1980's and uses low cost materials, C and Ni-mesh. The AFC needs pure gases in input which limited their application and the further development of this technology.
The AFC are competitive with polymeric electrolyte fuel cells (PEFC). The AFC advantageously does not need the presence of costly separation diaphragms or membranes, avoiding the known problems arising from their degradation, and of noble metals catalyzed primary electrodes.
The alkaline fuel cells advantageously use low cost, carbon/nickel-mesh porous electrodes which can effectively be employed in a modified cell working as electrolyser.
The alkaline fuel cells are easily polluted from the carbon dioxide contained in the hydrogen produced from the hydro-reforming of the fossil fuels. Such a problem does not exist when the hydrogen is produced from the water hydrolysis. The hydrogen can be then used in a fuel cell producing electric energy and closing the energy cycle of transformation of energy from electric energy into chemical energy and from chemical energy to electric energy with a total energy yield above the 50%.
The alkaline fuel cell are the type of fuel cells with higher yield, up to 65%, and able to work from room temperature up to 200° C. and at pressure up to 200 bar: this high flexibility allows the choice of the most suitable operative conditions either for optimize the total yields or for reduce the complexity and cost of the plants.